System and method for vapor pressure controlled growth of infrared chalcogenide glasses

ABSTRACT

A system and method for preparing chalcogenide glass are provided that allow for larger quantities of glass to be produced with lower production costs and less risks of environmental hazards. The system includes a reaction container operable to hold chalcogenide glass constituents during a glass formation reaction, a stirring rod operable to mix the contents of the reaction container, a thermocouple operable to measure the temperature inside the reaction container, and a reaction chamber operable to hold the reaction container. The method includes placing chalcogenide glass constituents in a reaction container, heating the chalcogenide glass constituents above the melting point of at least one of the constituents, promoting dissolving or reaction of the other constituents, stirring the reaction melt, maintaining an overpressure of at least one atmosphere over the reaction melt, and cooling the reaction melt to below the chalcogenide glass transition temperature.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to the field of optics and, inparticular, to a system and method for the vapor pressure controlledgrowth of infrared chalcogenide glasses.

BACKGROUND OF THE INVENTION

Chalcogenide glasses, those containing one or more of the chalcogenideelements sulfur, selenium, or tellurium, are used in a variety ofoptical applications. Because of their excellent infrared transmittalproperties and relatively low production costs, these glasses,especially selenium-based glasses, are commonly used in infrared opticalsystems, such as thermal imaging and night vision systems.

Currently, most chalcogenide glasses are produced as small boules insealed quartz reaction containers. This helps minimize the loss ofselenium, which is prone to evaporate out of the reaction melt duringthe glass formation reaction due to its relatively high vapor pressure.However, production processes such as these suffer from high productioncosts due to the consumption of the quartz reaction containers, longproduction times, and environmental risks due to possible explosions.

Once the glass is formed, any practical optical application requiresthat the glass be formed into an optical component, such as a lens.Current production processes for such optics rely mainly on grinding ormolding the chalcogenide glass into lenses. These, however, are ratherlengthy processes. After the glass is formed from its constituentelements in a quartz reaction container, the glass is cast into a plateand annealed, so as to avoid breakage. The annealed plate is then cutinto blanks, which are ground to thickness, edged, and turned or groundinto lenses or formed into lenses in a vacuum press. This process cantake several days to complete. Furthermore, production volumes areconstrained by the size of the mold ovens used to produce the lenses.

The production of chalcogenide lenses is also hampered by the molds usedto form the lenses. Molds typically used for such molding are relativelycomplex, having a large number of parts and requiring measuring andshimming each time the molds are disassembled for cleaning. Because oftheir high part count, the tolerance stack-up for each mold preventsmolds from being built with acceptable tolerances for high yieldprocesses. Furthermore, reassembly of the molds adds much variability tothe part tolerance stack-up, as well.

The molds also suffer from a variety of mechanical problems, as variousmold components fail due to the high temperatures they are exposed toduring casting and molding. Often times threaded fasteners employed inthe molds fail or gall at high temperatures. Furthermore, galling andfriction with the mold guide pins frequently lead to mold closingfailures.

SUMMARY OF THE INVENTION

In accordance with the present invention, a system and method forpreparing chalcogenide glass are provided. The system comprises areaction container operable to hold chalcogenide glass constituentsduring a glass formation reaction, a stirring rod operable to mix thecontents of the reaction container, a thermocouple operable to measurethe temperature inside the reaction container, and a reaction chamberoperable to hold the reaction container. The method comprises placingchalcogenide glass constituents in a reaction container, heating thechalcogenide glass constituents above the melting point of at least oneof the constituents, stirring the reaction constituents, maintaining anoverpressure of at least one atmosphere over the reaction melt, andquenching the reaction melt to below the chalcogenide glass temperature,such as by pouring the glass into a plate or other desired shape.

A technical advantage of particular embodiments of the present inventionincludes the ability to produce chalcogenide glasses in largequantities, while minimizing production costs and environmental hazardsfrom production.

Another technical advantage of particular embodiments of the presentinvention is that the reaction container is not consumed in thechalcogenide production process. Instead, it may be reused for severalproduction cycles, helping to reduce the cost of chalcogenideproduction.

Yet another technical advantage of particular embodiments of the presentinvention is a reduction in the possibility of a runaway reaction thatcould lead to an explosion that could have both commercial andenvironmental consequences.

Other technical advantages will be readily apparent to one skilled inthe art from the following figures, descriptions and claims. Moreover,while specific advantages have been enumerated above, variousembodiments may include all, some, or none of the enumerated advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, and for furtherfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 illustrates an apparatus for forming chalcogenide glass from itsconstituent elements;

FIG. 2 illustrates a flowchart of the process of forming chalcogenideglass from its constituent elements;

FIG. 3 illustrates an isometric view of a mold assembly used for castingchalcogenide lenses;

FIG. 4 illustrates a cut-away view of a mold assembly used for castingchalcogenide lenses;

FIG. 5A illustrates an isometric view of a mold and clamp assembly usedfor casting chalcogenide lenses that is in the closed position;

FIG. 5B illustrates a side view of a mold and clamp assembly used forcasting chalcogenide lenses that is in the closed position;

FIG. 6A illustrates an isometric view of a mold and clamp assembly usedfor casting chalcogenide lenses that is in the open position;

FIG. 6B illustrates a side view of a mold and clamp assembly used forcasting chalcogenide lenses that is in the open position; and

FIG. 7 illustrates an isometric view of an automated system for castingchalcogenide lenses;

FIG. 8 illustrates a top view of an automated system for castingchalcogenide lenses;

FIG. 9 illustrates a side view of a casting chamber used in an automatedsystem for casting chalcogenide lenses;

FIG. 10 illustrates a side view of a mold press used in an automatedsystem for casting chalcogenide lenses;

FIG. 11 illustrates a flowchart of a method for using an automatedsystem for casting chalcogenide lenses; and

FIG. 12 illustrates the temperature evolution typical during achalcogenide glass reaction.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an apparatus that accommodates a method for vaporpressure controlled growth of chalcogenide glass, in accordance with aparticular embodiment of the present invention. Chalcogenide glassesformed using the apparatus and techniques described herein may be usedto form optical components that may be used in many fields, for example,infrared imaging. Such chalcogenide glasses are compounded from theirelemental constituents by reacting them to a molten state and quenchingthe resultant melt to below its glass transition temperature.

With respect to ternary and quaternary glasses based upon selenium, suchas Ge—Sb—Se and/or Ge—Sn—Sb—Se, care must be taken to minimize seleniumloss due to evaporation. In accordance with the present invention, oneor more of five controlling factors may be used to minimize seleniumloss: overpressure, melt surface baffling, a hot-wall reaction vessel,heat balance control during phase changes, and reaction control bymixing rate control. Other methods to minimize selenium loss may also beused within the teachings of the present invention.

In FIG. 1, chalcogenide glass is formed in reaction container 10, whichis housed within externally heated, hot wall reaction chamber 11.Typically reaction container 10 is constructed of quartz. However, othersuitable materials, such as ceramics or pyrolytic graphite could be usedas well. Within reaction container 10, selenium, germanium, and antimonyare reacted to form a ternary chalcogenide glass. These constituentelements are represented in FIG. 1 by Se piece 14, Sb piece 13, and Gepiece 12. Although not shown, tin could also be added to form aquaternary glass.

These constituents, referred to collectively as reaction melt 21, aremixed by stirring rod 16 within reaction container 10. In accordancewith a particular embodiment of the present invention, the temperatureof reaction melt 21 may be monitored by melt thermocouple 17, or byother methods such as an optical pyrometer or by fluorescence decayoptical sensors.

In addition to providing adequate mixing, stirring rod 16 is also usedto control the rate of reaction within the reaction melt 21. Duringglass formation, several temperature spikes are developed in response tothe evolved heat of reaction. As these spikes are detected by meltthermocouple 17, the rotation rate of stirring rod 16 is reduced by anappropriate amount to arrest the rate of the temperature increase.

Particular embodiments of the reaction container also feature valve arm18. Valve arm 18 features a wing-type or shear-type valve device. Unlikea ball-type valve, which could introduce air pockets to the moltenglass, a wing-type or shear-type valve slides from side-to-side to openand close the valve. This prevents the introduction of air pockets andfacilitates precision metering of glass. Furthermore, as valve arm 18couples with reaction container 10 to seal valve 19 against hole 22, thevalve arm 18, valve seat 19, and reaction container 10 are typicallyconstructed from similar materials to minimize thermal expansioneffects, and valve seal 19 and the surface surrounding the hole 22 areground flat and smooth to function as a liquid tight seal.

As mentioned above, a variety of measures are taken to reduce the lossof selenium or other volatile constituents out of reaction melt 21 dueto evaporation, in various embodiments of the present invention. Onesuch measure is the maintenance of an overpressure of at least oneatmosphere over reaction melt 21 in space 20. In a particularembodiment, this is accomplished by introducing an inert gas, such asnitrogen, helium, or argon, into reaction container 10 and maintaining apressure higher than the vapor pressure of the most volatileconstituent. In this embodiment, the most volatile constituent isselenium.

The mass transfer of selenium (or other volatile constituents) may alsobe controlled at the surface of reaction melt 21 by mass boundary layer15. Similar to a liquid encapsulation reaction (LER), mass boundarylayer affects molecular streaming, diffusion, and convection over thereaction melt 21, serving as a physical barrier to the transfer ofselenium out of reaction melt 21. One method of establishing such alayer is by introducing a non-reactive material, such as boron oxide(B₂O₃), that has a melting point lower than the boiling point ofselenium and a density lower than that of selenium. Another method is tointroduce quartz plates over the reaction melt 21, either floating onthe melt surface or placed at the top of reaction container 10.

Another method for minimizing selenium loss employed by particularembodiments of the reaction apparatus is the use of hot-wall reactionchamber 11. The use of hot-wall reaction chamber 11 allows the glassformation to take place under isothermal conditions and prevents thewalls of reaction chamber 11 from serving as condensation areas forselenium vapors that escape from reaction melt 21. Reaction chamber 11can also be lined with materials that are chemically inert to selenium,including a nickel-chromium-iron alloy, such as Inconel®, to preventselenium loss due to any reactions with the walls of reaction chamber 11that could form metal selenides. In addition, reaction chamber 11 may beexternally heated, prolonging heater life and reducing production costs.

Yet another way selenium loss is minimized in particular embodiments ofthe reaction apparatus is by using heat balance control during phasechanges within reaction melt 21. As mentioned earlier, there are variousexothermic effects during glass formation that result in abrupttemperature increases. FIG. 12 illustrates the temperature evolutiontypical during chalcogenide glass manufacturing, including two distincttemperature spikes 181 and 182 followed by a series of smallertemperature spikes 183. The first of these temperature spikes, spike181, at approximately 70° C. and 2 atm, is due at least in part toselenium sintering. The next spike, spike 182, at approximately 500° C.and 2 atm, is due at least in part to the latent heat of crystallizationof a solid compound (probably antimony selenide (Sb₂Se₃)) formed priorto glass formation. Finally, a series of smaller spikes 183 around 700°C. and 2 atm are due at least in part to the heat of reaction of Ge withthe rest of the elements in reaction melt 21. The temperature ofreaction melt 21 is adjusted in response to each of these temperaturespikes 181-183, by adjusting the power to the heaters heating reactionchamber 11 using specific algorithms for each temperature profile. Inthis way, these exothermic heats of reaction are used to propel thevarious steps of the chalcogenide glass reaction, while minimizing thepossibility of having a potentially disastrous runaway reaction develop.

First, the selenium sintering temperature spike, spike 181, is exploitedto increase the heating rate and bring the temperature quickly above theselenium melting point.

The crystallization temperature spike, spike 182, is detected bymonitoring the time derivative of the temperature reported by meltthermocouple 17 near the temperature at which the spike begins(approximately 500° C. at 2 atm). During the upward temperature surge,the heater power is reduced. The size of the temperature spike 182 isalso reduced by using large pieces of Sb, thus presenting a smallerreaction interface to the Se melt (larger pieces have less surface areaper volume than smaller pieces). This is illustrated in FIG. 1 by thedisparate sizes of Se piece 14 and Sb piece 13.

Once the temperature begins to fall after temperature spike 182, theheater power is restored to remelt the solidified material. Completeremelting is then detected by monitoring the rotation of and/or torqueapplied to stirring rod 16. When stirring rod 16 rotates freely insidereaction melt 21, its rotation rate is increased to a maximum value tomix reaction melt 21 and promote the glass formation reaction.

In response to the small temperature spikes due to the heat of reactionof Ge with the rest of the elements in reaction melt 21, spikes 183, themixing rate of stirring rod 16 is also adjusted. As these temperaturespikes 183 are detected by melt thermocouple 17, the rotation rate ofstirring rod 16 is reduced by an appropriate amount to at leastpartially arrest the rapid temperature increase.

Once the glass formation reaction is complete and no more temperatureripples are observed, the heater power is reduced and reaction melt 21is brought down to an acceptable pouring temperature (i.e., atemperature where the glass is easily poured yet does not splatter).After reaction melt 21 has adequately cooled, valve arm 18 is released,pouring the liquid glass through valve 19 where it is allowed tocontinue to cool. Pouring the glass into another container at a lowertemperature facilitates a rapid temperature change, thus quenching thematerial.

Various chalcogenide glasses can be made using an apparatus such as thatdescribed in FIG. 1. Two such glasses are Ge₁₂Sn₇Sb₁₃Se₆₈ (LCG111) andGe₂₈Sb₁₂Se₆₈ (TI 1173). The compositions for these two glasses are givenin the following table.

Ge₁₂Sn₇Sb₁₃Se₆₈ (LCG111) Ge₂₈Sb₁₂Se₆₀ (TI 1173) Glass Composition GlassComposition Element Mole % Weight % Mole % Weight % Ge 12 10 28 24.7 Sn7 9.6 — — Sb 13 18.3 12 17.7 Se 68 62.1 60 57.6

In reacting these constituents to form a chalcogenide glass, theelemental constituents are loaded into a quartz reaction container.Typically, the constituents are layered in the reaction container fromtop to bottom as follows: Sb, Se, Ge, Se, and Sn (if applicable). Noticethat the layer of Ge is disposed between two layers of Se, the twolayers dividing between them the total weight of selenium in thereaction melt.

The constituent layers are comprised of various size particles,depending on the elemental constituent. These particle sizes, as well asthe amount of each elemental constituents used in each layer in both 5kg and 10 kg batches, are shown in the table below.

Ge₁₂Sn₇Sb₁₃Se₆₈ Ge₂₈Sb₁₂Se₆₀ (LCG111) (TI 1173) Piece 5 kg 10 kg 5 kg 10kg Element Size Batch Batch Batch Batch Sb 1″ × 2″  914 g 1828 g 1234.5g 2469 g Chunks Se Shot 1601 g 3202 g   1489 g 2978 g Ge 1″ × ½″  503 g1006 g  887.5 g 1775 g Chunks Se Shot 1601 g 3202 g   1489 g 2978 g SnShot  480 g  960 g — —

Of course, these tables, and their associated descriptions herein, areprovided for illustration and example only. Other chalcogenide glasseshave different compositions and reaction conditions. It will berecognized by those of ordinary skill in the art that various techniquesmay be employed in implementing the process disclosed herein, varyingwith glass compositions and reaction conditions.

An example of how an apparatus such as that shown in FIG. 1 is operatedis provided by FIG. 2, which illustrates a flowchart of a process formaking one type of chalcogenide glass, LCG111.

This process begins with a vacuum bakeout in block 202. In the vacuumbakeout, the walls of the reaction chamber are heated to a predeterminedtemperature set point, such as 100° C., and the chamber is pumped downto a vacuum, such as a pressure of 100 mTorr or less. This process maybe aided by conducting multiple pump and nitrogen backfills, which helpto reduce the amount of oxygen and water vapor in the chamber.

Once the temperature indicated by the melt thermocouple is sufficientlywarm (e.g., in this example 60° C.), a first heating stage is started inblock 203. In this first heating stage, the chamber wall temperature isset to temperature sufficient to initiate Se sintering, and the nitrogenpressure is set to provide an overpressure of at least one atmosphere.In this example, a wall temperature set point of 250° C. and a nitrogenpressure set point of approximately 1520 Torr (or 2 atm) are used. Theseconditions are maintained for 150 minutes, until the first exothermicreaction temperature spike, due to Se sintering, is detected in block204.

After the first temperature spike, a second heating stage begins inblock 205. In this second heating stage the reaction chamber set pointis set to a temperature sufficient to initiate the Sb₂Se₃crystallization reaction, such as 580° C., which results in a secondtemperature spike (due to the Sb₂Se₃ crystallization) in block 206.

After the second temperature spike is detected, the reaction chamber setpoint is set a temperature sufficient to initiate the Ge reaction in athird heating stage in block 207. In this example, 725° C. is used.

Block 208 then confirms that the temperature spikes are complete. Thisassurance is provided when the melt temperature has reached 675° C., orsome other predetermined temperature near the desired temperature setpoint, and the melt thermocouple ramp rate is sufficiently low, such asless than 1.0° C./min.

The reaction melt is then mixed with a stirring rod to assure adequatemixing. Mixing is controlled by adjusting the rotation rate of thestirring rod. At high rotation rates the glass reaction rate may be toohigh, resulting in an increase in the temperature of the reaction meltdue to the exothermic heat of reaction. This could potentially increasethe Se vapor pressure, resulting in substantial Se loss. Therefore, themixing rate is closely controlled.

As part of this close control, the stirring rate is slowly increased inblock 209 (e.g., 1 RPM every 5 minutes), but only if the melt ramp rateis still sufficiently low, such as less than 1.1° C./min, and only ifthe melt thermocouple doesn't indicate a significant temperatureincrease (e.g., the melt temperature stays below 700° C.). This is doneuntil the stirring rate reaches 60 RPM in block 210.

Once block 210 confirms that the stirring rod has reached 60 RPM, thereaction melt is stirred at 60 RPM for 120 minutes in block 211. After120 minutes, the stirring rod is slowed to 20 RPM in block 212 byreducing the stirring rate 1 RPM every 10 seconds until a stirring rateof 20 RPM is achieved.

The reaction chamber set point is then set to 0° C. in block 213. Oncethe melt temperature approaches the appropriate pour temperature (e.g.,within 20° C. of the pour temperature), the mixture is allowed tofurther cool for a specified time, for example 5 minutes, while beingstirred to ensure its homogeneity, before it is poured in block 214. ForLCG111, the appropriate pour temperature is 560° C., but, of course,this temperature is composition dependent. To prevent crystallization,and at the same time allow the melt to flow and fill the receivingcontainer, the glass may be poured into a heated mold or tray, which inthis example is kept at a temperature of approximately 250° C.Otherwise, the glass may simply be poured into any shape of interest.

Following pouring, the process continues to cooldown in block 215.During this time the reaction chamber set point remains at roomtemperature and the pressure set point remains at 1520 Torr. Theseconditions are maintained for approximately 60 minutes. Afterapproximately 60 minutes, the material may be slow cooled using heaterpower to minimize stress as the glass cools through the glass transitiontemperature, after that all contactors are turned off, and the glass isallowed to free cool in block 216, before the process terminates inblock 217.

FIG. 2, and its associated descriptions herein, are provided forillustration and example only. Other chalcogenide glass formationreactions may have different temperature profiles and require differentreaction conditions. It will be recognized by those of ordinary skill inthe art that various techniques may be employed in implementing theprocess disclosed herein, varying with glass compositions and reactionconditions.

Once the chalcogenide glass has been formed, the glass may be cast,molded, or machined into shape. FIG. 3 illustrates an isometric view ofa mold assembly used for the casting or molding of chalcogenide opticalcomponents. Casting typically involves pouring liquid glass into a mold,and molding typically involves placing a sold perform into the mold.This mold assembly comprises mold halves 30 and 31. Each mold half 30and 31 defines the shape of one face of a lens. A taper and shoulder oneach mold half (not visible in this isometric view) provide precisecontrol of centering and perpendicularity of the two mold halves 30 and31. The mold assembly also includes three pins 32 a–32 c and springs(not illustrated in this isometric view), which are operable to hold themold open for casting.

FIG. 4, which illustrates a cut-away view of a mold assembly similar tothe one shown in FIG. 3, provides a better understanding of the designof the mold assembly. Like the mold illustrated in FIG. 3, the moldassembly illustrated in FIG. 4 is comprised of two mold halves 40 and41. Mold halves 40 and 41 are configured to be removably coupled suchthat a first face of mold half 40 and a second face of mold half 41 forman interface that defines a lens-shaped cavity 406. In this way, moldhalves 40 and 41 define the shape of a lens, which is typicallydescribed by mathematical equations, each mold half 40 or 41corresponding to one face of the lens.

To provide precise control of the centering and perpendicularity, moldhalves 40 and 41 also feature circumferential tapers 401 a and 401 b andshoulders 402 a and 402 b that mate to properly align the mold halves 40and 41. Tapers 401 a and 401 b ensure proper optical alignment forcentering, while shoulders 402 a and 402 b ensure the perpendicularityof the lenses formed. Unless properly centered and perpendicular, thelenses produced by the molds will have limited uses in opticalapplications. Therefore, tapers 401 a and 401 b and shoulders 402 a and402 b assure proper lens geometry by controlling centering andperpendicularity when the mold closes.

The mold assembly also features a circumferential, shaped reservoir 403located between lens-shaped cavity 406 and tapers 401 a and 401 b. Thisshaped reservoir 403 prevents the flow of molten glass from lens-shapedcavity 406 back into the locating tapers 401 a and 401 b, and yet allowsenough glass to be dispensed into the mold assembly to ensure that themold is adequately filled. Reservoir 403 also features vertical surface405, which further helps to prevent molten glass from flowing into thelocating tapers 401 a and 401 b by presenting a surface that is moredifficult to climb (relative to the tapered surface 404 located below).

Located at the base of circumferential reservoir 403, between thereservoir 403 and the lens-shaped cavity 406 is flash gap 47. As thename implies, flash gap 47 is a small gap between the area of the moldassembly that forms the lens and circumferential reservoir 403. This gapallows molten glass to flow from lens-shaped cavity 406 up intoreservoir 403. Flash gap 47 also helps provide an edge detail on thelens formed by the mold assembly. As flash gap 47 provides only a narrowpassage way between the lens face of the mold and reservoir 403, uponcooling, the glass in flash gap 47 suffers a stress fracture along thegap. This leaves an edge detail on the lens formed by the mold assembly.This edge detail can be useful in the mounting of the lens.

The mold assembly also,features a plurality of pins and springs locatedbetween the two mold halves 40 and 41, operable to hold the mold halvesopen for casting. These are illustrated by pins 42 a and 42 b, andspring 44 a coupled with pin 42 a (additional springs and pins may beincluded, but are not illustrated in this figure). In FIG. 1, pin 42 aand spring 44 a are shown sitting within a pocket within reservoir 403;however, in other embodiments, the pins and springs may be locatedelsewhere between mold halves 40 and 41. Typically, three sets of pinsand springs are used, as three pins and springs provide a determinatestructural arrangement. Additional or fewer pins and springs could beused. However, for most operations, a determinate number of pins andsprings is usually desirable.

Mold half 40 also features plurality of inlet holes, illustrated byinlet hole 43. Although not illustrated in this figure, other inletholes may also be included (see inlet holes 33 a–33 c in FIG. 3).Although not required for lens molding, this feature may be used as aninlet to the mold when casting lenses. In fact, although having only oneinlet hole may be adequate to fill the mold, additional inlet holeslocated symmetrically around mold half 40 may provide the added benefitof yielding a weight balanced mold.

As shown in FIG. 4, inlet hole 43 feeds directly in reservoir 403. Whenglass is poured through inlet hole 43, the glass falls on tapered ramp404, which makes up part of the reservoir 403. Pouring onto this taperedsurface helps prevent the molten chalcogenide glass from splatteringwhen it comes in contact with mold half 41.

FIG. 4 also shows vent hole 46 located in mold half 41. Although notrequired for casting, vent hole 46 accommodates the venting of air whenthe mold is used in a puck molding process. This prevents theaccumulation of air pockets on the face of a lens that is being formed.Although vent hole 46 is shown located in mold half 41, additional oralternate vent holes could be located in mold half 40, as well,depending on the geometry of the lens being formed by the mold assembly.

Mold half 41 also features a plurality of mold handling holes,illustrated by mold handling holes 45 a and 45 b (other mold handlingholes may be included, but are not illustrated). These shallow pin holesallow for the automated handling of the mold assembly, especially underhigh temperature conditions. Mold handling holes 45 a and 45 b couldalso accommodate a thermocouple, in addition to serving a mold handlingpurpose.

The two-piece mold design described above offers several technicaladvantages. One such advantage is that it uses far fewer parts thanprevious molds. Some previous molds used as many as 92 different parts.The two-piece design significantly lowers that part count.

Another technical advantage of particular embodiments of the mold is theability to machine all the critical surfaces of the mold, including thelens faces, lens edges, taper, and shoulder in one setup of a diamondpoint turning (DPT) lathe. This results in the ability to control thecentering and perpendicularity of the lens without any appreciabletolerance stack-up.

The simple tooling is also easy to disassemble and clean in an aqueoussystem. Furthermore, the lack of any blind, small, or trapped holesprevents the trapping of fluid during cleaning. The design also does notrequire the use of fasteners, which could fail after several hightemperature process cycles.

Additionally, the two-piece design is also smaller and lighter thanprevious molds, resulting in a mold having less thermal mass.

However, to take advantage of these benefits and yet consistentlyproduce quality lenses, the construction material of the mold assemblymust be chosen carefully. As the chalcogenide lens and mold expand andcontract as they are heated and cooled, it is important to select a moldmaterial that has a coefficient of thermal expansion similar to that ofthe chalcogenide glass being formed. Otherwise, as the lens and moldexpand and contract at different rates, the lens could potentially bedamaged. One example of such a mold material that has an acceptablecoefficient of thermal expansion is hardened 420 stainless steel. Asliquid chalcogenide glass can erode tool steel up to ⅛ inch deep in onecasting cycle, the hardened 420 stainless steel mold may be coated witha surface coating that can protect the mold surfaces, and yet notdegrade the optical surfaces of the lens. An example of such a coatingthat works well with hardened 420 stainless steel is titanium nitride(TiN). Additional protection may be supplied by coating the mold with amold release compound, such as Kisscote 1086™.

Of course, even with these materials, it is still typical in the moldingand casting of lenses to cast a lens with a slightly different shapethan that which is ultimately desired. This is done to compensate forshrinkage and distortion of the lens as it cools so that the ultimateproduct has the exact optical qualities desired.

A clamp assembly may also be added to the mold assembly to hold the moldopen for casting and closed for molding. Such a clamp assembly isillustrated in FIGS. 5A–6B. Although not instrumental to the mold, theabove mentioned pins and springs, along with such clamp assembly aredesigned to allow fully automated disassembly, cleaning and reassemblyof the mold.

FIG. 5A illustrates an isometric view of a mold and clamp assembly thatis used for casting chalcogenide lenses. This mold is comprised of moldhalves 50 and 51. Clamp assembly 55, comprising clamp assembly arms 56a–56 c, is operable to hold mold halves 50 and 51 closed. This isaccomplished by clamp assembly arms 56 a–56 c, which extend down thesides of mold halves 50 and 51, and lock in place on lip 57 on mold half51. In this position, clamp assembly 55 compresses the springs alongpins 52 a–52 c, holding mold halves 50 and 51 together. In this closedposition arms 56 a–56 c are preloaded to overcome springs 44 a–44 c andkeep the mold halves together after the press operation. In this way,arms 56 a–56 c press down on mold halves 50 and 51 from above. A sideview of this assembly shown in FIG. 5B.

FIGS. 6A–6B illustrate the same mold and clamp assembly shown in FIG.5A–5B. However, the mold illustrated in FIGS. 6A–6B is in the openposition.

FIG. 6A illustrates an isometric view of the mold assembly comprised ofmold halves 50 and 51 and clamp assembly 55. In this figure, clampassembly 55, rather than holding mold halves 50 and 51 together in theclosed position, holds mold halves 50 and 51 apart in the open position.In this open position, clamp assembly arms 56 a–56 c extend down thesides of mold halves 50 and 51, locking in place in grove 58 in the sideof mold half 51. In this position the springs along pins 52 a–52 c areallowed to expand, forcing apart mold halves 50 and 51. A side view ofthis open mold and clamp assembly is shown in FIG. 6B.

One technical advantage of molds such as those illustrated in FIGS. 3–6Bis that they may be used in an automated casting system, allowing forgreater automation of the chalcogenide lens production process andlarger production runs.

FIG. 7 illustrates an isometric view of such an automated casting systemfor casting chalcogenide lenses. This automated casting system iscomprised of two main sections, mold chamber 70 and casting chamber 71.Inside mold chamber 70, empty molds are heated to a temperature abovethe melting temperature of the chalcogenide glass. The empty molds arethen transferred to casting chamber 71, where liquid chalcogenide glassis cast into the molds. After being filled with glass, the molds arereturned to mold chamber 70, where the molds are pressed closed andallowed to cool. By isolating casting chamber 71 from mold chamber 70,the possibility of a casting failure ruining a large batch of lenses ordamaging mold chamber 70 is greatly reduced.

The construction of mold chamber 70 and casting chamber 71 also offersseveral benefits. Both mold chamber 70 and casting chamber 71 feature ahot wall construction. This results in a lower thermal mass, uniformheating, longer heater life, and minimum volatile condensation on theinterior walls of mold chamber 70 and casting chamber 71. Theconstruction of mold chamber 70 is also easily scalable, which allowsthe capacity of the automated casting system to be increased by simplyincreasing the length-of mold chamber 70.

Separating mold chamber 70 and casting chamber 71 is pressure isolationvalve 72. Separating mold chamber 70 and casting chamber 71 withpressure isolation valve 72 allows the two chambers to be loaded,evacuated, and heated independently. During casting, however, pressureisolation valve 72 is open, allowing molds to be moved from mold chamber70 to casting chamber 71 to be filled with liquid chalcogenide glass.

Molds are loaded into mold chamber 70 through loading stations 76 b.Once inside mold chamber 70, the molds are moved through the chamber ina closed loop. This is accomplished by mold pushers 74 a–75 b. Moldpushers 74 a and 74 b work in concert to move the molds in one plane,while mold pushers 75 a and 75 b work in concert to move the molds inanother, perpendicular plane.

FIG. 7 also illustrates melt chamber 73. This chamber is similar to theapparatus for forming chalcogenide glass shown in FIG. 1. Integratingmelt chamber 73 into the automated casting system allows more of theproduction of chalcogenide lenses to be combined into one machine, wherethe constituent elements of chalcogenide glass are loaded into theautomated casting system and chalcogenide lenses are output. This isknown as a “rocks in, lenses out” process, where the automated systemreacts chalcogenide glass, homogenizes the glass, transfers the glassinto molds, presses the lenses to shape, and anneals the glass. Althoughnot essential to the automated casting system, this integration allowsgreater optimization of production requirements. In a non-integratedprocess, melt chamber 73 could instead be loaded with pre-reactedchalcogenide glass to be cast into molds in casting chamber 71.

A better understanding of the operation of the automated casting systemis available from FIG. 8, which illustrates a cut-away, top view of anautomated casting system similar to that shown in FIG. 7.

The automated casting system shown in FIG. 8 is comprised of moldchamber 80 and casting chamber 81. These two chambers are separated bypressure isolation valve 82.

Open molds are loaded in mold chamber 80 on 3-by-1 trays through loadingstations 86 b. As with the molds, the choice of a tray material is alsoimportant (though to a lesser extent), as the trays tend to gall anddegrade after repeated exposure to the extreme temperatures inside themold chamber. Therefore, the trays are constructed of a material able towithstand repeated exposure to these temperatures without prematurelydegrading. One such material that has proven adequate for this purposeis H13, a hardenable stainless steel alloy, although other materials maybe used as well.

Once the trays of molds are loaded into the mold chamber, the trays arethen moved counter-clockwise through mold chamber 80 by mold pushers 84a–85 b. Mold pushers 84 a–84 b work in concert to move the trays in oneplane, while mold pushers 85 a–85 b work in concert to move the trays inanother, perpendicular plane. Although other mechanisms, such as belts,rollers, or walking beams, may be used to move the molds within the moldchamber, the four-point pusher system of mold pushers 84 a–85 b offersthe added benefits of reduced costs and enhanced scalability.

Once the molds reach the end of mold chamber 80 adjacent to castingchamber 81, transport arm 87 reaches through casting chamber 81 andpressure isolation valve 82 to grab a single mold. This mold is thenbrought back through pressure isolation valve 82 into casting chamber81, where it is filled with liquid chalcogenide glass. Once filled, themold is then returned to its tray inside mold chamber 80.

After casting, the molds continue to move in the closed loop in moldchamber 80. At the end of mold chamber 80 opposite casting chamber 81,the filled molds are closed by mold press 88. The molds continue alongthe closed loop until all molds are closed. Once close, the molds areallowed to cool until they are cool enough to be removed.

FIG. 9 illustrates casting chamber 91, which may be used in a particularembodiment of an automated casting system. Empty molds are brought intocasting chamber 91 by transport arm 97, which is operable to retrieve asingle empty mold through pressure isolation valve 92. Once insidecasting chamber 91, the empty mold is placed on casting tray 94. Castingtray 94 positions the empty mold beneath pour spout 96, through whichliquid chalcogenide glass is dispensed into the empty mold from meltchamber 93, which is constructed similar to the device shown in FIG. 1.Like the casting chamber and mold chamber, pour spout 96 may also beheated. By constructing the entire automated casting system withexternally-heated hot walls, there are fewer chances of localizedhot/cold spots and less opportunities for materials to condense on theinterior walls of the automated casting system.

Beneath casting tray 94 is dump tray 95. Dump tray 95 helps prevent anysystem damage due to casting failures. In the event of such a castingfailure, dump tray 95 serves as a collection point for any excesschalcogenide glass, protecting the rest of the machine from the glass,which could potentially damage the machine and would require anextensive clean-up before any further production runs could becompleted. The mold lift mechanism supports the mold during casting, andsupports the dump tray. A loadcell within the lift mechanism senseschanges in weight of the dumptray and the mold. The loadcell is used tomeasure the amount of glass dispensed into a mold. The loadcell is alsoused to measure the amount of glass dispensed during practice dispenses,into the dump tray before casting into molds. The dispense algorithmdetermines the amount of time the dispense valve is to be opened. Thevalve open time for the next dispense is adjusted based on analysis ofthe several previous dispense weights and times. The loadcell alsosenses any leakage from the melt chamber to allow early intervention.

Dump tray 95 is also useful in learning to dispense molten glass intothe molds in casting chamber 91. As the viscosity of molten chalcogenideglass is very sensitive to temperature changes, the flow characteristicsof the glass can change during casting. Therefore, it is often useful topractice and/or calibrate the dispensing of molten glass by pouring ontodump tray 95 instead of into a mold.

As mentioned above, a mold press may be used to close the molds aftercasting. FIG. 10 illustrates such a mold press, mold press 108, used inan automated casting system. In mold press 108, mold press arm 101engages the bottom of a mold that is to be closed and presses the openmold against mold press plate 102, forcing the mold into the closedposition.

A process for using an automated casting system similar to that shown inFIGS. 7–10 is illustrated in FIG. 11. In step 111, the melt chamber isloaded with the constituent elements of the chalcogenide glass beingproduced. These elements are reacted in step 112 to form liquidchalcogenide glass.

Empty molds are prepared in step 113, including coating the moldsurfaces with a mold release compound. These prepared molds are thenloaded into the mold chamber in step 114.

The mold chamber undergoes a vacuum bakeout at approx 100° C. and 3mTorr to reduce the oxygen (O₂) and water vapor (H₂O) concentrationswithin the chamber to below 100 ppm and 500 ppm, respectively, and thenthe chamber and molds are heated to a temperature above the meltingtemperature of the chalcogenide glass, typically 300° C., all in step115. Glass is then cast into the molds in step 116. After the molds havebeen filled with liquid chalcogenide glass, the molds are closed by themold press in step 117.

Following pressing, the mold chamber and casting chamber are cooled instep 118, during which time the power to the heater is reduced,typically cooling the chamber to approximately 200° C. Cooling rates arecontrolled with heater power to minimize thermal stress on the hot wallchambers, and to reduce stress in the glass as it cools through theglass transition temperature. At lower temperatures (step 119) thecooling rate is accelerated by force cooling the heaters and moldchamber and casting chamber. In this process, forced air is blownbetween the heaters and the walls of the chambers to speed theircooling.

After the molds have adequately cooled, the molds are removed from themold chamber in step 120, and the lenses are removed from the molds instep 121. Afterwards, the molds and melt chamber are cleaned in step122, in preparation for another run when the production cycle restartsin step 111.

Although embodiments of the invention and their advantages are describedin detail, a person skilled in the art could make various alterations,additions, and omissions without departing from the spirit and scope ofthe present invention as defined by the appended claims.

1. A method for preparing chalcogenide glass, comprising: placing aplurality of constituents of a chalcogenide glass in a reactioncontainer, wherein each constituent has a respective melting point,boiling point, density, and vapor pressure, and wherein the chalcogenideglass has a glass transition temperature; heating the constituents abovethe melting point of at least one of the plurality of constituents toform a reaction melt; stirring the reaction melt; maintaining anoverpressure of at least one atmosphere over the reaction melt; andcooling the reaction melt to below the glass transition temperature ofthe chalcogenide glass.
 2. The method of claim 1, wherein theconstituents are selected from the group consisting of selenium,germanium, antimony, and tin.
 3. The method of claim 1, furthercomprising maintaining a pressure that is higher than the vapor pressureof the constituent having the highest vapor pressure at a currentprocess temperature.
 4. The method of claim 1, further comprisingintroducing one or more inert gases into the reaction container in aquantity sufficient to maintain the overpressure.
 5. The method of claim4, wherein the inert gas is selected from the group consisting ofnitrogen, helium, or argon.
 6. The method of claim 1, further comprisingintroducing a non-reactive material into the reaction container to atleast partially control mass transfer of the constituents out of thereaction melt; wherein a melting point of the non-reactive material isless than each of the respective boiling points of the constituents; andwherein a density of the non-reactive material is less than each of therespective densities of the constituents.
 7. The method of claim 6,wherein the non-reactive material includes boron oxide (B₂O₃).
 8. Themethod of claim 1, further comprising disposing a quartz plate, or othernon-reactive material above the reaction melt to control mass transfer,of the constituent having the lowest boiling point, out of the reactionmelt.
 9. The method of claim 1, further comprising placing the reactioncontainer within an externally heated, hot-wall reaction chamber. 10.The method of claim 9, wherein the reaction chamber includes anickel-chromium-iron alloy liner.
 11. The method of claim 10, whereinthe nickel-chromium-iron alloy liner includes Inconel®.
 12. The methodof claim 9, wherein heating the constituents comprises providing heat tothe reaction chamber.
 13. The method of claim 12, further comprising:monitoring a temperature of the reaction melt; and adjusting the heatsupplied to the reaction chamber according to the temperature of thereaction melt.
 14. The method of claim 13, wherein adjusting the heatsupplied to the reaction chamber includes adjusting power supplied to anexternal heater operable to heat the reaction chamber.
 15. The method ofclaim 12, further comprising: monitoring exothermic heats of reactiongiven off by the reaction melt; and adjusting the heat supplied to thereaction chamber according to the monitored exothermic heats ofreaction.
 16. The method of claim 15, wherein adjusting the heatsupplied to the reaction chamber includes adjusting power supplied to anexternal heater operable to heat the reaction chamber.
 17. The method ofclaim 1, wherein stirring the reaction melt includes mechanicallyrotating a stirring rod positioned at least partially within thereaction melt; and wherein the stirring rod is rotated at a stirringrate.
 18. The method of claim 17, further comprising: monitoring atemperature of the reaction melt; and adjusting the stirring rate of thestirring rod to minimize rapid temperature rises.
 19. The method ofclaim 17, further comprising: monitoring a torque applied to thestirring rod; and adjusting the stirring rate of the stirring rod inresponse to the torque detected.
 20. The method of claim 1, furthercomprising: monitoring a temperature of the reaction melt; detectingtemperature spikes by calculating time derivatives of the temperature ofthe reaction melt; and adjusting the temperature of the reaction melt inresponse to the temperature spikes.
 21. The method of claim 1, furthercomprising providing constituents having large volume to surface arearatios in order to present a relatively small reaction interface. 22.The method of claim 1, wherein the reaction container comprises a quartzcontainer being disposed within a closed, pressurized, externallyheated, hot-wall reaction chamber.